One of my nuts is missing

I don't know but that type of nut comes in 1mm increments so it's easy enough to measure with anything, a ruler will do.

Just measure the diameter and then it's the 'M' type, so if the bolt is say 5mm then get a M5 nut from any hardware shop.

I can’t help you with the nut but from that photograph your rim is very worn and needs checking professionally. Worn rims and high pressure tyres are a safety concern, if you’re lucky you’ll get a warning when your brakes start rubbing, if you’re unlucky the rim will go pop and the tyre deflate rapidly. I’ve seen plenty of examples where the rim has split, in some cases cutting the rider from the razor sharp shards of metal. Please get it checked.

your rim is very worn and needs checking professionally

Username checks out

Ossify: thanks for that - I've got to admit to having been a bit lazy and hoping that someone already knew the measurements 🙂  I'll measure it up tonight

Wheelsonfire1: interesting - it's an el-cheapo from Superstar which I've used through 3 winters (all conditions), and at a guess approx 4-5k miles.  I guess you might be right.

Well, they do seem to come in pairs.  But, it has to be said, seem a bit like a design afterthought.

This is a very worn rim that demonstrates how the pressure from the tyre acts on the wall of the rim with potentially catastrophic consequences!

I may have exploded a rear rim on my commuter - used to get just 18 months from a rim on the rear.

I may have exploded a rear rim on my commuter – used to get just 18 months from a rim on the rear.

I had that on my Surly.  The reason was that I was using the Tektro cantilever brakes which squealed like and absolute banshee.  I got used to only using my front brake in emergencies and when I wanted to let people know I was annoyed at their driving/walking, relying on the rear brake for all other braking.

I switched to Vs after I realised replacing rear rims was going to be a twice a year job if I carried on like that.

In case it helps anyone else, it's a 4mm M6 lock nut

That's a very worrying photo that wheelsonfire posted - had me rushing off and checking all my rims asap...then I remembered they're all disc brakes now!